PROJECT SUMMARY Multiple myeloma (MM) is an incurable blood cancer and is the second most common hematologic cancer. The majority of patients die within 5 years of diagnosis, hence there is an urgent need to develop drugs with new mechanisms of action. Viral oncolytic therapy with Reolysin, the infusible form of human reovirus (RV), is supported by extensive preclinical data in myeloma with its antitumor activity driven by a combination of direct cytolysis and immune responses against infected MM cells. Relapsed MM patients in our phase 1 single agent RV trial tolerated the treatment well and the RV selectively entered MM cancer cells. However productive viral infection associated with tumor cell death was not seen. Our subsequent phase 1b trial of RV plus the proteasome inhibitor (PI) Carfilzomib (CFZ) demonstrated objective responses in 11 of 12 evaluable patients, an effect that could not be achieved by CFZ alone. Importantly, MM patients enrolled in our phase 1b were also resistant to the PI bortezomib (BTZ) and hence their objective response to RV+CFZ is a very encouraging. It's not known whether the enhanced RV activity in combination with chemotherapeutic agents is due to increased productive infection, augmentation of the immune response, or both. The long-term goal of this project is to optimize the anti-cancer activity of RV in MM patients. Our overall objective, which is the next step toward attainment of our goal, is to increase RV killing of MM cells by enhancing MM cell permissiveness to RV. Our central hypothesis is that RV clinical efficacy will be potentiated by the addition of CFZ and/or HDACi due to their ability to increase RV accessibility and infection efficiency of cancer cells. We will test our central hypothesis and achieve our goals through the following aims: (1) Test the safety and efficacy of Reolysin with the proteasome inhibitor Carfilzomib in a phase 1b trial in relapsed MM patients; (2) Assess the role of proteasome inhibition in modulating the innate immune response and increase killing of RV infected MM cells; and (3) Measure tumor sensitivity to the addition of HDACi to RVPI in vitro and in NOD-SCID mice. We will confirm that Reolysin-based regimens will enhance not only productive viral infection in MM cells, but transiently suppress infiltrating cytotoxic T-lymphocytes and suppress the innate antiviral IFN response. With completion of this work, we will have introduced an entirely novel, highly effective treatment and provided mechanistic data to build upon its success in combination.